Polyether Ligands Research Articles

Helical polyether-immobilized chiral aza-bis(oxazolines): Synthesis and synergistic effect on the enantioselectivity of Zn-catalyzed Henry reaction

Novel polyether-immobilized aza-bis(oxazolines) ligands derived from optically pure epichlorohydrin and aza(bisoxazolines) were prepared by anionic polymerization. Through chiral optical studies, it is determined that the chiral polyether ligands can maintain stable helical conformation in the solvent, with controllable molecular weights and low polydispersity index of 1.02–1.06. Asymmetric Henry reactions of aldehydes were performed to investigate the relationship between the catalytic activity and the helical conformation of these polyethers, gave the adducts in up to 97 % yield with high enantiomeric excess(98 % ee). In particular, there is a synergistic effect between the helical chirality of the polyether backbone and the point chirality of the pendant aza-bis(oxazolines) in the asymmetric Herny reaction. Helical polyether-immobilized chiral aza-bis(oxazolines) ligands can be easily recycled at least 10 times without losing their enantioselectivity. It was also found by MALDI-TOF-MS that zinc(II) combined with polyether probably result in a double spiral species in the presence of more than 2.5 mol% Zn(OAc)2, which lead to negative effect on catalytic performance of the polyether.

Proton conduction in hydronium solvate ionic liquids affected by ligand shape.

We investigated the ligand dependence of the proton conduction of hydronium solvate ionic liquids (ILs), consisting of a hydronium ion (H3O+), polyether ligands, and a bis[(trifluoromethyl)sulfonyl]amide anion (Tf2N-; Tf = CF3SO2). The ligands were changed from previously reported 18-crown-6 (18C6) to other cyclic or acyclic polyethers, namely, dicyclohexano-18-crown-6 (Dh18C6), benzo-18-crown-6 (B18C6) and pentaethylene glycol dimethyl ether (G5). Pulsed-field gradient spin echo nuclear magnetic resonance results revealed that the protons of H3O+ move faster than those of cyclic 18C6-based ligands but as fast as those of acyclic G5 ligands. Based on these results and density functional theory calculations, we propose that the coordination of a cyclic ether ligand to the H3O+ ion is essential for fast proton conduction in hydronium solvate ILs. Our results attract special interest for many electro- and bio-chemical applications such as electrolyte systems for fuel cells and artificial ion channels for biological cells.

Determining the Coordination Number of Li+ and Glyme or Poly(ethylene glycol) in Solution Using Attenuated Total Reflectance-Far Ultraviolet Spectroscopy.

Attenuated total reflectance-far ultraviolet (ATR-FUV) spectra of Li+ and polyether ligands, such as glymes and poly (ethylene glycol) (PEG), in solution give information about changes in the electronic states of the ligands. From the ATR-FUV spectra, the coordination numbers between Li+ and monoglyme, diglyme, triglyme, and PEG400 were determined to be 4, 5, 6, and 5, respectively. Our results indicate that Li+ is coordinated only by the ligands rather than its counter-ions.

ChemInform Abstract: Metallacrown Ether Catalysts Containing Phosphine‐Phosphite Polyether Ligands for Rh‐Catalyzed Asymmetric Hydrogenation — Enhancements in Activity and Enantioselectivity.

AbstractNovel polyEther podand ligands containing R‐BINOL and triphenylphosphine derived terminal oxygen substituents are developed and successfully applied as ligands for the asymmetric Rh‐catalyzed hydrogenation of α‐arylenamides.

Metallacrown Ether Catalysts Containing Phosphine–Phosphite Polyether Ligands for Rh‐Catalyzed Asymmetric Hydrogenation – Enhancements in Activity and Enantioselectivity

AbstractA new class of tunable metallacrown ether rhodium catalysts based on α,ω‐(phosphine–phosphite) polyether ligands were prepared either by a template‐induced method or by a nontemplate procedure. For the asymmetric hydrogenation of α‐arylenamides with the addition of K+ cations, the ortho‐diphenylphosphine‐substituted metallacrown ether catalyst showed high enantioselectivities (up to 99 % ee), which are comparable to or better than the results obtained for phosphine–phosphite ligands with two or more chiral elements. Remarkable enhancements in enantioselectivity and noticeably increased catalytic activity were achieved through the supramolecular recognition between K+ cations and the metallacrown ether catalyst.

A Comparative Study of (Poly)ether Adducts of Alkaline Earth Iodides – An Overview Including New Compounds

AbstractNew homoligand and mixed‐ligand adducts of the heavier alkaline earth metal (Ca, Sr, Ba) halides with oxygen‐donor polyether ligands have been isolated and characterized and are compared with previously obtained compounds of the same class in order to give an overview on structures and properties. Homoligand halide adducts, discussed herein, are [CaI(DME)3]I (1), trans‐[SrI2(DME)3] (2), trans‐[BaI2(DME)3] (3), (DME = ethylene glycol dimethyl ether), [CaI(diglyme)2]I (4), cis‐[SrI2(diglyme)2] (5), trans‐[BaI2(diglyme)2] (6),(diglyme = diethylene glycol dimethyl ether, [SrI(triglyme)2]I (7), and [BaI(triglyme)2]I (8), (triglyme = triethylene glycol dimethyl ether). Introduction of the mono‐coordinating THF ligand (THF = tetrahydrofuran) in the coordination sphere of 1, 2, 3, 4 allows the formation of the new mixed‐ligand compounds trans‐[CaI2(DME)2(THF)] (9), trans‐[SrI2(DME)2(THF)] (10), trans‐[BaI2(DME)2(THF)2] (11), and trans‐[CaI2(diglyme)2(THF)2] (12). These compounds were obtained from the metal halide salts in solution with pure or mixtures of ether solvents. While compounds 1–8 appear to be very stable and non‐reactive, adducts 9–12 present a comparable reactivity to the well known THF adducts [MI2(thf)n] (M = Ca, n = 4; Sr, Ba, n = 5).

Experimental and Computational Insights into the Stabilization of Low-Valent Main Group Elements Using Crown Ethers and Related Ligands

A series of tin(II) triflate and chloride salts in which the cations are complexed by either cyclic or acyclic polyether ligands and which have well-characterized single-crystal X-ray structures are investigated using a variety of experimental and computational techniques. Mössbauer spectroscopy illustrates that the triflate salts tend to have valence electrons with higher s-character, and solid-state NMR spectroscopy reveals marked differences between superficially similar triflate and chloride salts. Cyclic voltammetry investigations of the triflate salts corroborate the results of the Mössbauer and NMR spectroscopy and reveal substantial steric and electronic effects for the different polyether ligands. MP2 and DFT calculations provide insight into the effects of ligands and substituents on the stability and reactivity of the low-valent metal atom. Overall, the investigations reveal the existence of more substantial binding between tin and chlorine in comparison to the triflate substituent and provide a rationale for the considerably increased reactivity of the chloride salts.

Generally Applicable NMR Titration Methods for the Determination of Equilibrium Constants for Coordination Complexes: Syntheses and Characterizations of Metallacrown Ethers with α,ω-Bis(phosphite)-polyether Ligands and Determination of Equilibrium Binding Constants to Li+

A new metallacrown ether, cis-PdCl2{(2,2′-C12H8O2)P(OCH2CH2)4OP(2,2′-O2H8C12)}, has been synthesized and fully characterized by NMR spectroscopy and X-ray crystallography. A new synthetic method and complete NMR characterization for the previously reported and closely related cis-Mo(CO)4(2,2′-C12H8O2)P(OCH2CH2)4OP(2,2′-O2H8C12) metallacrown ether are also reported. Both complexes are monomeric, exhibiting cis coordination geometries both in solution and in the solid state, and the cis-Mo(CO)4(2,2′-C12H8O2)P(OCH2CH2)4OP(2,2′-O2H8C12) metallacrown ether does not appear to undergo cis–trans isomerization in the presence of HgCl2. These results are surprising because the closely related PdCl2(Ph2P(CH2CH2O)nCH2CH2PPh2) (n = 3–5) metallacrown ethers exhibit both cis–trans and monomer–cyclic oligomer equilibria in solution, and cis-Mo(CO)4 (Ph2P(CH2CH2O)nCH2CH2PPh2) (n = 3–5) metallacrown ethers rapidly undergo cis–trans isomerization in the presence of HgCl2. The coordination of Li+ cations to the cis-Mo(CO)4(2...

Coordination and morphology of metal/polyetherurethane complexes

Based on infrared spectra, differential scanning calorimetry, and quantum chemical calculations, this study investigated the effects of the species of metal (MnCl 2, FeCl 3, CoCl 2, CuCl 2, GaCl 3), their interaction sites and binding strength on the morphologies of polyetherurethane (PU). The calculated frequency and binding energy were consistent with the experimental results, suggesting that most of the binding strength (95%) is associated with the charge transfer from the lone pairs (on the nitrogen in NH and the oxygen in CO and COC) in the PU ligand to the metal. MnCl 2, CoCl 2, and CuCl 2 are more soluble in soft segments of PU than GaCl 3 and FeCl 3 via coordination with one or two polyether ligands and can significantly raise the glass transition temperature of PU. Conversely, GaCl 3 and FeCl 3 interact more strongly with hard-segment NH of PU than MnCl 2, CoCl 2, and CuCl 2 and markedly reduce the melting temperature of PU.

The Syntheses and Characterization of Three Novel Macrocyclic Polyether Ligands and Some of Their Alkaline Metal Complexes

The macrocyclic ligands 2,3,5,6-Bis{3'-bromo-5'-tert-butyl benzo-5-pheny l benzo} 12C3 (L1), 2,3,5,6-bis{3 '-bromo-5'-tert butyl-benzo-5'-phenyl-benzo}15C4 (L 2), 2,3,5,6-bis{3 '- bromo-5'-tert butyl-benzo-5'-phenyl-benzo}18C5 (L 3) have been synthesized and their alkaline metal compounds with NaClO4·H2O and KClO4 have been prepared. Chemical formulas of all compounds have been characterized by using elemental analyses and spectroscopic techniques.

Flexible cyclic ethers/polyethers as novel P2-ligands for HIV-1 protease inhibitors: design, synthesis, biological evaluation, and protein-ligand X-ray studies.

We report the design, synthesis, and biological evaluation of a series of novel HIV-1 protease inhibitors. The inhibitors incorporate stereochemically defined flexible cyclic ethers/polyethers as high affinity P2-ligands. Inhibitors containing small ring 1,3-dioxacycloalkanes have shown potent enzyme inhibitory and antiviral activity. Inhibitors 3d and 3h are the most active inhibitors. Inhibitor 3d maintains excellent potency against a variety of multi-PI-resistant clinical strains. Our structure-activity studies indicate that the ring size, stereochemistry, and position of oxygens are important for the observed activity. Optically active synthesis of 1,3-dioxepan-5-ol along with the syntheses of various cyclic ether and polyether ligands have been described. A protein-ligand X-ray crystal structure of 3d-bound HIV-1 protease was determined. The structure revealed that the P2-ligand makes extensive interactions including hydrogen bonding with the protease backbone in the S2-site. In addition, the P2-ligand in 3d forms a unique water-mediated interaction with the NH of Gly-48.

Structural and luminescence studies of trivalent europium complexes with pentaethylene glycol and 18-crown-6 ligands in the presence of picrate anion

Two new isostructural complexes of europium picrate (Eu-Pic) with pentaethylene glycol (EO5) and 18-crown-6 (18C6) ligands formed complexes of molecular formula [Eu(Pic) 2(18C6)] +(Pic) − I and [Eu(Pic) 2(EO5)] +(Pic) − II have been isolated and characterised. Compound I showed 10-coordination number through six oxygen atoms from the 18C6 ligand and two bidentate picrate anions. Meanwhile, compound II exhibited 9-coordination number via six oxygen atoms from EO5 ligand, two oxygen atoms from a bidentate and one oxygen atom from monodentate picrate anions. Photoluminescence (PL) spectra of the solid-state europium complexes display sharp lines which are assigned to 5D 0→ 7F 0–4 and 5D 1→ 7F 1,2,4 transitions. No emission of polyether ligands is observed, indicating that the energy transfer from the polyether ligands to the Eu 3+ ion is quite efficient. The PL spectra of [Eu(Pic) 2(OH 2) 6] +(Pic) −·6H 2O III, [Eu(NO 3) 3(OH 2) 3]·(18C6) IV, [Eu(NO 3) 3·6H 2O] V and Eu 2O 3 VI are also observed. Compounds I– IV exhibited high Ω 2 intensity parameter values, namely 16.93, 10.23, 17.10 and 12.35 (in units of 10 −20 cm 2), respectively. These relatively high values reflect the hypersensitive behaviour of the 5D 0→ 7F 2 transition and indicate that the Eu 3+ ion is located in a highly polarisable chemical environment.

Intermolecular Electron-Transfer Mechanisms via Quantitative Structures and Ion-Pair Equilibria for Self-Exchange of Anionic (Dinitrobenzenide) Donors

Definitive X-ray structures of "separated" versus "contact" ion pairs, together with their spectral (UV-NIR, ESR) characterizations, provide the quantitative basis for evaluating the complex equilibria and intrinsic (self-exchange) electron-transfer rates for the potassium salts of p-dinitrobenzene radical anion (DNB(-)). Three principal types of ion pairs, K(L)(+)DNB(-), are designated as Classes S, M, and C via the specific ligation of K(+) with different macrocyclic polyether ligands (L). For Class S, the self-exchange rate constant for the separated ion pair (SIP) is essentially the same as that of the "free" anion, and we conclude that dinitrobenzenide reactivity is unaffected when the interionic distance in the separated ion pair is r(SIP) > or =6 Angstroms. For Class M, the dynamic equilibrium between the contact ion pair (with r(CIP) = 2.7 Angstroms) and its separated ion pair is quantitatively evaluated, and the rather minor fraction of SIP is nonetheless the principal contributor to the overall electron-transfer kinetics. For Class C, the SIP rate is limited by the slow rate of CIP right arrow over left arrow SIP interconversion, and the self-exchange proceeds via the contact ion pair by default. Theoretically, the electron-transfer rate constant for the separated ion pair is well-accommodated by the Marcus/Sutin two-state formulation when the precursor in Scheme 2 is identified as the "separated" inner-sphere complex (IS(SIP)) of cofacial DNB(-)/DNB dyads. By contrast, the significantly slower rate of self-exchange via the contact ion pair requires an associative mechanism (Scheme 3) in which the electron-transfer rate is strongly governed by cationic mobility of K(L)(+) within the "contact" precursor complex (IS(CIP)) according to the kinetics in Scheme 4.

Anion-Templated Assembly of Pseudorotaxanes: Importance of Anion Template, Strength of Ion-Pair Thread Association, and Macrocycle Ring Size

A wide range of pseudorotaxane assemblies containing positively charged pyridinium, pyridinium nicotinamide, imidazolium, benzimidazolium and guanidinium threading components, and macrocyclic isophthalamide polyether ligands have been prepared using a general anion templation procedure. In noncompetitive solvent media, coupling halide anion recognition by a macrocyclic ligand with ion-pairing between the halide anion and a strongly associated cation provides the driving force for interpenetration. Extensive solution 1H NMR binding studies, thermodynamic investigations, and single-crystal X-ray structure determinations reveal that the nature of the halide anion template, strength of the ion-pairing between the anion template and the cationic threading component, and to a lesser extent favorable second sphere pi-pi aromatic stacking interactions between the positively charged threading component and macrocyclic ligand, together with macrocyclic ring size, affect the efficacy of pseudorotaxane formation.

“Separated” versus “Contact” Ion-Pair Structures in Solution from Their Crystalline States: Dynamic Effects on Dinitrobenzenide as a Mixed-Valence Anion

Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline (reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB(-)) can be prepared in the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of macrocyclic polyether ligands: L(C) (cryptand) and L(E) (crown-ethers). The crystalline "separated" ion-pair salt isolated as K(L(C))(+)//DNB(-) is crystallographically differentiated from the "contact" ion-pair salt isolated as K(L(E))(+)DNB(-) by their distinctive interionic separations. Spectral analysis reveals pronounced near-IR absorptions arising from intervalence transitions that characterize dinitrobenzenide to be a prototypical mixed-valence anion. Most importantly, the unique patterns of vibronic (fine-structure) progressions that also distinguish the "separated" from the "contact" ion pair in the crystalline solid state are the same as those dissolved into THF solvent and ensure that the same X-ray structures persist in solution. Moreover, these distinctive NIR patterns are assigned with the aid of Marcus-Hush (two-state) theory to the "separated"ion pair in which the unpaired electron is equally delocalized between both NO(2)-centers in the symmetric ground state of dinitrobenzenide, and by contrast, the asymmetric electron distribution inherent to "contact"ion pairs favors only that single NO(2)-center intimately paired to the counterion. The labilities of these dynamic ion pairs in solution are thoroughly elucidated by temperature-dependent ESR spectral changes that provide intimate details of facile isomerizations, ionic separations, and counterion-mediated exchanges.

Synthesis and Spectroscopic Studies of Some New Polyether Ligands of the Schiff Base Type

New polyether ligands of Schiff base type (3–13) were synthesized from the reaction of diethylene glycol bis(2‐aminophenyl)ether and triethylene glycol bis(2‐aminophenyl)ether with salicylaldehyde, 5‐methoxysalicylaldehyde, 5‐bromosalicylaldehyde, 5‐nitro salicylaldehyde, and 2‐hydroxy‐1‐naphthaldehyde. The products were characterized by elemental analysis, IR, 1H, 13C NMR, and UV‐VIS techniques. The UV‐VIS spectra of those Schiff bases with an OH group in the ortho position to the imino group were studied in polar and nonpolar solvents in acidic and basic media. The compounds are in tautomeric equilibrium (enol‐imine, O–H · N⇌keto‐amine, O · H–N forms) in solvents, acidic chloroform, and benzene solutions and basic DMSO, chloroform, and benzene solutions. These tautomers were not observed in polar and non‐polar solvents and in basic solutions of DMSO, chloroform, and benzene for the Schiff bases 5–10. Tautomer proportions, which were obtained from 1H NMR and UV‐VIS data in DMSO, were compared for compounds 3, 4, 11, and 12.

Ring opening reactions of 1,2-epoxides catalyzed by complexes of polyether ligands with metal halides in low polarity solvents. Comparison with heterogeneous systems

Metal ion electrophilic catalysis has been revealed in ring opening reactions of 1,2-epoxides 1-5 promoted by complexes of polyethers 6-9 with alkali metal halides MHal (M + = Li + , Na + , K + ; Hal - = I - , Br - ) in solvents of low polarity (chlorobenzene, 1,2-dichlorobenzene). The catalytic effect depends on the Lewis acid character of the cation (K + cyclic polyether). Results have been rationalized on the basis of a transition state where the complexed cation (M + ⊂ Lig) stabilizes the negative charge developing on the oxygen atom of the oxirane ring while favors the nucleophilic attack at the adjacent carbon by the halide anion Hal - .

Polar Molecular Precursors for Alkali and Alkaline Earth Metal Clusters and Low‐Dimensional Polymer Structures: the Solid‐State Structures of [CaI(dme)3]I, and cis‐[SrI2(diglyme)2] (dme = CH3OC2H4OCH3; diglyme = CH3(OC2H4)2OCH3)

AbstractTwo new alkaline earth metal halide adducts with oxygen‐donor polyether ligands are described. One of them, cis[SrI2(diglyme)2], features two terminally bonded anions in vicinal positions, and at an angle of ca. 90°. The low bond valence sum of ca. 1.9 indicates how well the cation is shielded by its ligands in the solid state, and that ligand exchange can be expected in solution and upon reaction. The second compound, [CaI(dme)3]I, consists of an ion pair formed by the cationic complex [CaI(dme)3]+ with one anion in terminal position, and one iodide. Both compounds possess a strong dipole moment at the molecular level, but crystallize in centrosymmetric space groups. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Demethylation reactions of phosphate esters catalysed by complexes of polyether ligands with metal iodides

Metal ion catalysis has been observed in demethylation reactions of methyl di(para-substituted phenyl)phosphate esters 1–4 promoted by complexes of polyether ligands 5–7 with alkali metal iodides MI (M = Li, Na, K) in low polarity media (chlorobenzene, 1,2-dichlorobenzene). The catalytic effect is found to depend on both the metal ion charge density and the ligand topology. Rate constants increase, in the order K+ < Na+ < Li+, with the complexes of crown ether 5 and PEG 6, whereas they are much lower and independent of the cation with the cryptates of 7. Reaction rates are favored by electron-withdrawing substituents, with Hammett ρ-values that increase with decreasing Lewis acid character of the cation (Li+ < Na+ < K+) or when the charge of the latter is shielded by a good complexing agent like cryptand 7. Kinetic data are explained on the basis of a concerted “push–pull” mechanism.

Dramatic effect of the metal cation in dealkylation reactions of phosphinic esters promoted by complexes of polyether ligands with metal iodides.

Metal ion electrophilic catalysis has been revealed in dealkylation reactions of phosphinic esters 1-4 promoted by complexes of polyether ligands 5-7 with metal iodides MI(n) (M[n+] = Li(+), Na(+), K(+), Rb(+), Ca(2+), Sr(2+), Ba(2+)) in low polarity solvents (chlorobenzene, 1,2-dichlorobenzene, and toluene) at 60 degrees C. The catalytic effect increases with increasing the Lewis acid character of the cation, in the order Rb(+)< K(+)< Na(+)< Li(+) and Ba(2+)< Sr(2+)< Ca(2+). The results are interpreted in terms of a transition state where the complexed cation (M[n+] subset Lig) assists the departure of the leaving group Ph(2)P(O)O(-) and, at the same time, favors the attack at carbon of the nucleophile I(-) ("push-pull" mechanism). The rate sequence found for 1-4 (Me > Et >> i-Pr and t-Bu) shows that this reaction can be utilized for the selective dealkylation of these substrates.